The invention relates to electrochemical fuel cell stacks, and, more particularly to fuel cell stacks having improved assemblies.
An electrochemical fuel cell converts the chemical bond energy potential of fuel to electrical energy in the form of direct current (DC) electricity. Fuel cells are presently being considered as replacement for battery storage systems and conventional electric generating equipment.
An electrochemical fuel cell stack is formed of a plurality of individual fuel cells, each possessing a positive (+) and a negative (xe2x88x92) electrical pole, arranged in an electrical series relationship to produce higher useable DC voltage. A DC/AC inverter may be utilized to convert the DC electrical current to AC electrical current for use in common electrical equipment.
A stack of repetitive fuel cells connected in series requires additional non-repetitive hardware in order to operate. For example, manifolds or housings to control and direct reactant gasses, terminals to conduct product electricity, end cells to terminate the repetitive cells of the stack, compression system to apply sealing force, and dielectrics to prevent short circuits may be required.
Commonly, a fuel cell stack is enclosed within a housing for the purpose of controlling the unintended release of reactant and product gasses, and in some instances to provide a portion of a conduit that delivers the reactant gasses from their source to the fuel cell stack and/or product gasses from the fuel cell stack to a point of exhaust or recycle. U.S. Pat. No. 5,688,610 to Spaeh et al. describes a housing that controls the delivery of oxidant to the cathode inlet face of the fuel cell stack. The housing of Spaeh et al. is thermally insulated to control the release of thermal energy to the ambient environment. However, the housing of Spaeh et al. does not completely eliminate the manifolding requirements. Additional manifolds are required to direct the fuel to the stack and the spent fuel exhaust to the point of recycle. An oxidant outlet manifold is required to direct the spent oxidant to the point of exhaust or recycle. These additional manifolds require seals that are subject to leakage and contribute to the migration of electrolyte in liquid electrolyte fuel cell stacks.
U.S. Pat. No. 4,714,661 to Kaun et al. teaches an insulated housing enclosing a fuel cell stack. The housing further provides for the penetration of contact points for the current terminals of the fuel cell stack. A fuel cell stack is equipped with electrical current terminals to withdraw and return the electrical current generated by the electrochemical fuel cell reaction. A first electrical current terminal is typically placed in contact with the positive pole (+) of the first cell. A second electrical current terminal is typically placed in contact with the negative pole (xe2x88x92) of the last cell of the stack. A DC positive and a DC negative current terminal are thereby created.
Typically, a current terminal is comprised of a sheet or plate of electrically conductive material that extends in coplanar fashion with, and across the width and breadth of, the cells comprising the fuel cell stack. The current terminals may be combined with the end plates that are used to provide uniform application of a compressive force applied to the fuel cell stack.
An additional area of the sheet or plate of the current terminal provides a contact point for attachment of an electrical conductor. The contact point may extend beyond the periphery of, or above the plane of, the end cell, and may penetrate through thermal insulation that surrounds the fuel cell stack.
All fuel cells operate at temperatures above ambient room temperature. It is recognized that the maintenance of a uniform operating temperature of the individual cells of fuel cell stacks is critical for optimum performance, and the avoidance of distortions caused by differential thermal expansions. The electrically conductive current terminals create significant sources of heat loss for fuel cells designed to operate at high temperatures. In practical fuel cell stack designs, the current carrying capacities of the current terminals are balanced against the thermal losses created by the current terminals. Excessive heat loss through the terminals results in end cells that operate at less than optimum temperature, or that may have undesirable temperature gradients. The end cells may require additional heat input from electrical end cell heaters to maintain normal operating temperature.
It is desirable that the design and architecture of the end cells of a fuel cell stack be consistent with the design and architecture of central cells of the stack to provide continuity and uniformity of the mechanical stress that accumulates within the stack.
The hardware that comprises the end cells of the fuel cell stack includes the first and last repetitive bipolar separator plates of the stack, and the anode and cathode end plates. The end plates are monopolar plates that house the first anode and the last cathode electrodes, respectively. The first and last electrolyte membranes are installed within the end cells. The monopolar end plates should represent the design and architecture of the central cells because they convey the stack sealing force to the central cells of the stack. Deviation from the design and architecture of central cells results in mechanical discontinuities that contribute to undesirable stress and premature stack failure.
The compression system of the fuel cell stack is intended to uniformly apply the force that seals the individual cells and contributes to low electrical contact resistance at each interface of each component of each cell of the stack. The typical stack comprised of cells manufactured to specific cell area employs various methods of applying the stack compression force. One method, as taught by U.S. Pat. No. 6,057,053 to Gibb, utilizes tie rods that connect compression plates at opposing ends of the fuel cell stack. The rods may penetrate through the stack at apertures provided in the bipolar plates. Tension on the rods is adjustable and springs are provided to compensate for cell compression and thermal expansion. Another method utilizes load beams that traverse the compression plates, and may be cantilevered beyond the periphery of the cells comprising the fuel cell stack. Tie rods are again utilized between the load beams to apply the compressive force.
A dielectric insulator is utilized to electrically isolate one or both end cells from the compression system and/or the stack housing. The dielectric insulator extends to the periphery of the cells comprising the fuel cell stack. The dielectric strength of the insulator must be sufficiently high to prevent excessive short-circuiting of stack current. Low temperature fuel cells may utilize a wide variety of materials for constructing the insulator. The material of construction for the insulator of high temperature fuel cells is highly restricted. Commonly, alumina or mica is utilized as the material of construction for the insulator of high temperature fuel cells. Molten Carbonate Fuel Cell (MCFC) stacks have utilized thick, solid, cast alumina dielectric end plate insulators in conjunction with thick end compression plates. These assemblies are rigid and do not conform well to the changing dimensions of the fuel cell stack. Furthermore, high temperature differentials across the thickness of such end plates results in distortions of the end plates. Often, uniform electrical contact is not maintained at each interface of each cell component of each cell of the fuel cell stack.
U.S. Pat. No. 5,009,968 to Guthrie et al. teaches the use of a thin membrane end plate that will not distort when subjected to thermal differentials across the membrane thickness, since the induced stresses are insufficient to overcome the stack compressive forces. A resilient pressure pad in the form of insulation minimizes the effect of distortion of the thicker pressure plates that are exterior to the thermal insulation and less subjected to thermal distortion.
A phenomenon associated with MCFC stacks is the loss of electrolyte over time. Losses can occur through several mechanisms such as evaporation into the reactant gas streams, voltage driven migration, and simple film creepage. Evaporative losses can be limited by careful control of the reactant flow rate and composition. Voltage driven losses occur primarily through the manifold seals that traverse each cell at the sealing surfaces of each external manifold of externally manifolded fuel cell stacks. U.S. Pat. No. 5,110,692 to Farooque et al. teaches a method of disrupting the flow of voltage driven electrolyte through the manifold gasket. However, the flow of electrolyte is not completely inhibited by the design taught by Farooque et al. Over the practical life of the fuel cell, excessive quantities of electrolyte will migrate through the seals, resulting in an imbalance of electrolyte that impacts stack performance. Loss of electrolyte due to surface creepage can only be addressed by the significant reduction of available surface area.
Fuel cell stacks that utilize a liquid electrolyte may suffer from the undesirable migration of the electrolyte from the electrolyte membrane to other areas of the fuel cell. It is well known in the art that molten carbonate electrolyte employed in molten carbonate fuel cells has several mechanisms of migration. One such mechanism involves migration of the electrolyte via the seal of the external manifold housing. Typically, this seal is comprised of a solid dielectric material, as well as a caulk or paste applied to the irregular face of the stack of bipolar separator plates, and a porous gasket. During operation of the fuel cell stack liquid electrolyte migrates from the electrolyte membrane to the porous gasket and caulk. Voltage present in the fuel cell stack further results in migration of the electrolyte to the anode, or negative pole of the fuel cell stack. Electrolyte migration over time results in the flooding of cells with electrolyte at the anode, or negative pole, of the stack and results in an absence of electrolyte in the cells at the cathode, or positive pole, of the stack. This effect results in premature decay of cells at both poles of the fuel cell stack.
The prior art designs for the housing, end plates, current terminals, compression system, and dielectrics do not adequately address the demands of a stack of fuel cells constructed from such fuel cell sub-assemblies. Therefore, it is desirable to provide an improved fuel cell stack housing, end cell, compression system, dielectric insulator, and current terminals for fuel cell stacks utilizing a bipolar plate that is manufactured in fixed width and variable length. It is also desirable to provide an improved end cell/end plate/dielectric insulator/compression system assembly that offers high compliance to the dimensional changes that occur within the fuel cell stack. It is further desirable to provide a reduced quantity of external manifold seals and to reduce the surface area of the end cell hardware to reduce the losses of electrolyte that occur as a result of voltage driven migration and surface film creepage.
It is an object of the present invention to provide a fuel cell stack assembly that reduces or wholly overcomes some or all of the difficulties inherent in prior known devices. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this field of technology, in view of the following disclosure of the invention and detailed description of preferred embodiments.
The present invention provides an improved assembly for a fuel cell stack. Preferred embodiments of the present invention minimize the loss of electrolyte and reduce the materials required to form a fuel cell stack assembly.
In accordance with one aspect, a fuel cell apparatus includes a compression assembly, a housing contained entirely within the compression assembly, and a fuel cell stack positioned within the housing.
In accordance with another aspect, a fuel cell apparatus includes a housing comprised of a first half-shell and a second half-shell. A fuel cell stack is positioned within the housing. The fuel cell stack includes a plurality of bipolar separator plates positioned between a cathode end plate and an anode end plate. Each bipolar separator plate includes a plurality of segments, and each segment includes a fuel flow path and an oxidant flow path. Each of a plurality of anode compression bars is secured to a mating cathode compression bar. Each mating pair of compression bars is positioned between a pair of segments. The housing is positioned between the anode compression bars and the cathode compression bars. A cathode compression plate is positioned between each cathode compression bar and one of the half-shells. An anode compression plate is positioned between each anode compression bar and the other of the half-shells.